How do I control the blurriness of light through frosted glass?
Use a mix of glass and translucent shaders. To control how blurry the light source behind the glass is, play with the factor values on the mix and the roughness of the glass shader.

Here's an example of an emitter object enclosed on a glass cube. The glass walls  have to have some thickness (you can use a solidfy modifier) so that it is not a solid glass cube.



An other factor that will have a great impact in the light effects you are after, is the color transform used in the color management settings. Being able to use a bright light source and a much larger dynamic range for the image, will help you have a more realistic result. Experiment using the filmic blender colour tranforms.



(Click on the images to enlarge)

Read also:

Render with a wider dynamic range in cycles to produce photorealistic looking images